Cell-cell communication between endothelial cells (EC) and smooth muscle cells (SMC), provided by gap junctions, coordinates vessel contractility and electrical conduction. Gene knockout (KO) of vessel gap junction proteins (connexins (Cx)40 and Cx43) yields dramatically different blood pressures and conduction. A possible reason for these results are 1) co-regulation of Cx40 and Cx43, which in effect would 2) alter the method of cell-cell communication by gap junctions in the vessel wall. Based on this idea, I hypothesize that contact between EC and SMC plays a key role in modulating connexin function and expression, and thus in modulating vascular reactivity. In order to elucidate this hypothesis, an in vitro system to model EC and SMC interactions similar to in vivo will be developed and utilized. This model, in which EC and SMC cells will be isolated from the same mouse and aortic sections, will be used to test methods of dye and second messenger transfer between EC and SMC. These results will be compared to similar experiments on in situ wild type and connexin KO animals. Combined, these methods will help delineate the physiological role of connexins between EC and SMC.